Hierarchical nanostructures of diblock copolymer thin films directed by a saw-toothed substrate

Abstract

An extensive and systematic calculation was performed to explore hierarchical cylindrical structures and the order-to-order transitions of AB diblock copolymers (f(A) = 0.3) on a saw-toothed substrate using self-consistent mean-field theory. We obtained fifteen relatively simple morphologies, including the existing morphologies observed experimentally and from simulations, and five more complicated structures, by varying the lateral periodicity of the substrate, the film thickness of diblock copolymers, the interaction between the A-block and the substrate and the tilt angles (or the shape) of the substrate. These structures show that the orientation and number of layers of cylinders can be tailored. Even lamellae and spherical microdomains were observed. Most interestingly, hierarchical structures are also observed, such as the morphology of C(ab)(//) within the upper cylinder perpendicular to the bottom cylinder, SC(b)(//) morphology that the upper is a cylinder but the bottom is a sphere. In addition, we discussed these complex hierarchical structures in detail and have analyzed the order-to-order transitions between the cylindrical morphologies with distinct orientations and layers.

Non-native three-dimensional block copolymer morphologies

“…Field-theoretic methods have emerged as a powerful tool for understanding the assembly of BCP phases6, and it is likely that their application to the materials described herein could yield new insights. For instance, self-consistent field theory was used to demonstrate that relatively weak surface corrugations could strongly influence BCP assembly, selecting among different possible morphologies49. These simulations help to confirm the heuristic presented here.…”

“…BCP assembly is known to be responsive to confinement333435, guiding trenches363738 (i.e., grapho-epitaxy3940), topographic disruptions41424344, and substrate roughness4546, corrugations303132 or faceting474849. BCPs typically respond according to commensurability, reorienting and distorting to preserve their bulk equilibrium morphology.…”

Beyond native block copolymer morphologies

“…For instance, researchers have combined the complex morphologies of triblocks with structural control via confinement; 58,160 others have combined substrate patterning with tuning of film thickness to control morphology. 169 Further exploration of multi-modal strategies promises the discovery of many new self-assembled structural motifs. For instance, while triblock copolymers already broaden the bulk equilibrium morphological set substantially, additional structural motifs with a high degree of regularity may become possible as we deepen our understanding of fundamental aspects, such as polydispersity and hydrogen bonding in triblock blends.…”

“…morphologies with competing parallel and perpendicular orientations of morphology), hexagonally perforated lamellae, and hemispheres. 4,11,161,162,[164][165][166][167][168][169] The phase diagram for ordering within a thin film is remarkably rich, since conventional morphologies must frequently distort in order to accommodate the thickness dimension (Figure 4a). 170 Thus, one observes elongated spheres, squashed cylinders, hemi-cylinders, necked cylinders, crossed cylinders, 171 etc.…”

“…A simple example can be seen in the natural propensity for BCP line phases (horizontal cylinders or vertical lamellae) to orient orthogonal to underlying topographic undulations. 169,[214][215][216] Thus, a BCP ordering on top of an underlying BCP line pattern will generate local square or rectangular lattice symmetries, because of the natural tendency for the second layer to align orthogonally. 217 It is worth emphasizing that such symmetries are not natively seen in the diblock copolymer phase diagram.…”